1. Field of the Invention
The present invention relates to measures against electrode wear and improvement in ignition stability in a plasma ignition system used for ignition of an internal combustion engine.
2. Description of Related Art
In an internal combustion engine such as an automobile engine, a plasma ignition system 1x shown in FIG. 11A is known. In the system 1x, by applying high voltage between a center electrode 110x and a ground electrodes 130x of the plasma ignition plug 10x from a discharge power source 20x and by supplying a high current from a plasma generation power source 30x at the moment of the start of electric discharge in a discharge space 140x formed between the center electrode 110x and the ground electrode 130x, gas in the discharge space 140x is put into a plasma state of high-temperature and pressure and then the gas is injected from a leading end of the discharge space 140x so as to carry out ignition. Because the plasma ignition system 1x has good directivity and generates a very high temperature range from thousands to tens of thousands of degrees Kelvin (K) in a broad range in volume, the system 1x is expected to be applied as an ignition system for a lean burn engine having ignition resistance, such as homogeneous lean burn or stratified lean burn.
As a conventional technology of such a plasma ignition system, a surface gap spark plug is disclosed in U.S. Pat. No. 3,581,141 to prevent deterioration of the center electrode. The above surface gap spark plug includes a center electrode, an insulator having an insertion hole in its center, the hole holding the center electrode and extending longitudinally, a ground electrode, which covers the insulator and has an opening at its lower end, the opening communicating with the insertion hole, and a discharging gap, which is formed in the insertion hole.
Also, a technology which alms to lower discharge voltage is disclosed in JP-U-56-35793. According to the above technology, the discharge voltage is lowered by forming a projection or a recess, where an electric field density is locally high, at an end portion of a discharge surface of a center electrode.
However, in conventional plasma ignition systems such as U.S. Pat. No. 3,581,141 and JP-U-56-35793, the center electrode is used as a negative pole and the ground electrode is used as a positive pole. In this case, as in the case of the plasma ignition system 1x shown in FIG. 11B, cathode sputtering whereby the center electrode 110x is decomposed is easily generated, since a positive ion 50x having high temperature and large mass collides with a surface of the center electrode 110x. The surface of the center electrode 110x is heavily eroded due to the cathode sputtering. A discharge distance 141x between the center electrode 110x and the ground electrode 130x becomes gradually longer because of the erosion of the center electrode 110x. The discharge voltage rises gradually in proportion to the discharge distance 141x, and when the discharge voltage reaches a generated voltage of the discharge power source 20x or above in the course of time, electricity cannot be discharged and accordingly, there is a possibility of an accidental fire of the engine.
When the portion where the electric field density is locally high is formed on the surface of the center electrode through the formation of the projection or recess, as in the device in JP-U-56-35793, the center electrode still serves as a negative pole, so that the consumption of the center electrode due to the cathode sputtering is unavoidable, although an effect of reducing the discharge voltage is produced in its initial use. More specifically, the portion having the high electric field density is consumed first and consequently, the discharge voltage gradually rises. Eventually, there is a possibility of an accidental fire of the engine.
On the other hand, when the application of high voltage and the high current emission are performed on the inside of a certain discharge space, creeping discharge is generated to creep on a surface of an insulating member 120x, and gas around a creeping-discharge path is put into the plasma state. Because density of the gas in the plasma state immediately becomes high, further ionization of the gas becomes difficult despite the continuation of emission of electron. The volume of the discharge space needs to be enlarged in order to put more gas into the plasma state. However, according to the conventional configuration, when the volume of the discharge space is enlarged, the discharge distance becomes long, and accordingly discharge potential becomes high.
Furthermore, in stratified combustion of a lean mixture, accuracy in aiming the gas at a layer in the fuel/air mixture having high fuel concentration needs to be improved, by making an injection length of gas in the plasma state used as an ignition source as long as possible.